Techniques for extraction of metals from aqueous solutions obtained from ore or other sources are well-known. For example, an aqueous solution containing one or more metals of interest which originates from acid leaching of oxide ores, sulfide ores or lateritic ores may be contacted with an organic phase containing an extractant to concentrate the metal(s). More particularly, the extractant reacts with the metal to form a complex (“loading”) which is soluble in the organic phase. The extractant is chosen so that undesirable compounds, such as particular metals and other impurities, will not complex with it and remain in the aqueous phase (the “raffinate”) which is discarded or otherwise processed. The loaded organic phase may then be scrubbed to selectively remove further impurities. The metal(s) of interest is then obtained from the organic phase by stripping. Stripping typically involves reversing the reaction which caused the extractant to complex with the metal to produce an aqueous solution with even lower volume and more highly concentrated metal. The organic phase may then be subjected to a regeneration step which removes residual metals or impurities so that the organic phase can be recycled in the extraction process. Examples of processes for extraction are provided by U.S. Pat. Nos. 3,193,381, 5,447,552, 5,759,512 and 6,022,991, each incorporated herein by reference in their entireties.
A process for removal of ferric chloride from iron-aluminum chloride solutions and from iron-aluminum chloride solutions containing free hydrochloric acid using amines as liquid ion exchangers is described in U.S. Pat. No. 3,082,062. In one aspect, a cyclical process is described wherein a feed liquor which could optionally be FeCl3, AlCl3 and HCl, or just FeCl3 and AlCl3 is fed into an extractor. The FeCl3 is removed from the aqueous phase and transferred to an organic phase containing an amine hydrochloride. The FeCl3 is stripped from the organic phase and removed as an aqueous liquor which is substantially free of excess HCl. The amine hydrochloride, which is substantially free of iron, is then directed into the initial extractor.
A process for recovering gallium is described in U.S. Pat. No. 4,193,968. As described therein, an acid leach solution containing aluminum, ferric and gallium chlorides is treated by an amine ion exchanger which extracts the ferric and gallium values from the aluminum chloride solution. The amine may be a primary, secondary or tertiary amine which is dissolved in a water immiscible organic solvent that may also contain an alcohol. The organic phase containing the iron and gallium is separated from the aqueous phase which contains the aluminum chloride. The organic phase is then contacted with water or weak acid to strip it of iron and gallium. The resulting aqueous strip solution contains ferric and gallium chloride. The ferric ions are then converted to ferrous and gallium ions to gallium tetrachloride. The solution is then subjected to a second amine exchange which extracts only the gallium. The organic phase is stripped with either water or weak hydrochloric acid and then recycled to the extraction unit.
Extraction of nickel from acid leach solutions is described, for example, in U.S. Pat. No. 5,447,552. In accordance with that disclosure, a solvent extraction process may include a primary extraction step which involves contacting an aqueous leach solution containing Ni, Co, Zn, Mg, Mn, Ca, Fe and sulfate ions with an organic phase containing a dithiophosphinic extractant which selectively extracts Ni, Co, Fe and Zn. The aqueous solution containing Mg, Mn and Ca with sulfate anions (raffinate) may be separated from the organic phase and discarded. The Ni/Co/Zn/Fe loaded organic phase may then be contacted with an aqueous phase made acidic with a mineral acid to strip the Ni/Co/Zn/Fe from the organic phase into the acidic aqueous phase which is considered to be the product of the primary extraction phase. Zn may be removed from the aqueous primary extraction product by, e.g., ion exchange, which produces a product containing Ni/Co/Fe. Ni and Co may then be selectively removed from this product by means of a secondary extraction step using an amine solvent extractant, e.g., a tertiary amine in a water immiscible organic liquid. Ideally, Co is loaded onto the amine extractant and Ni is left in the acidic aqueous phase. The Co loaded amine may be contacted with a weak acidic aqueous solution to strip the Co from the organic phase and regenerate the amine solvent extractant for recycling into the secondary extraction step. U.S. Pat. No. 4,016,054 discloses that Co can be extracted from ferro-nickel using the tertiary amine triisooctylamine in an organic phase which is regenerated using a decinormal hydrochloric solution.
A problem with the secondary extraction step occurs due to the presence of Fe and trace amounts of Zn which pass through from the earlier extraction steps. Unfortunately, the Zn and Fe load strongly onto the amine extractant and are not removed from the organic phase as easily as Co. Accordingly, after repeated recycling of the amine extractant, there is a build-up of Zn and Fe in the organic phase. Such build-up interferes with loading of Co on the extractant and may eventually saturate the extractant, resulting in a loss of loading efficiency and eventually complete cessation of Co loading. One approach to dealing with this problem is to separately treat the amine extractant phase with a strong base solution to strip Zn and precipitate Fe as a hydroxide. See, e.g., U.S. Pat. No. 4,004,990. Unfortunately, this process results in precipitation of the Zn and Fe hydroxides as extremely fine particles which settle extremely slowly or not at all. As a result, a time consuming and relatively complicated filtration process is necessary to remove the fine solids.